1. Field of the Invention
This invention is generally related to fuel cell systems, and more particularly to controlling an output voltage of the fuel cell system.
2. Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant to electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which includes an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane electrode interface to induce the desired electrochemical reaction. In operation, the electrodes are electrically coupled for conducting electrons between the electrodes through an external circuit. Typically, a number of MEAs are electrically coupled in series to form a fuel cell stack having a desired power output.
In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have flow passages to direct fuel and oxidant to the electrodes, namely the anode and the cathode, respectively. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant, and provide channels for the removal of reaction products, such as water formed during fuel cell operation. The fuel cell system may use the reaction products in maintaining the reaction. For example, reaction water may be used for hydrating the ion exchange membrane and/or maintaining the temperature of the fuel cell stack.
Stack current is a direct function of the reactant flow, the stack current increasing with increasing reactant flow. The stack voltage varies inversely with respect to the stack current in a non-linear mathematical relationship. The relationship between stack voltage and stack current at a given flow of reactant is typically represented as a polarization curve for the fuel cell stack. A set or family of polarization curves can represent the stack voltage-current relationship at a variety of reactant flow rates.
In most applications, it is desirable to maintain an approximately constant voltage output from the fuel cell stack. One approach is to employ a battery in the fuel cell system to provide additional current when the demand of the load exceeds the output of the fuel cell stack. This approach often requires a separate battery charging supply to maintain the charge on the battery, introducing undesirable cost and complexity into the system. Attempts to place the battery in parallel with the fuel cell stack to eliminate the need for a separate battery charging supply raises additional problems. These problems may include, for example, preventing damage to the battery from overcharging, increasing efficiency, as well as the need for voltage, current, or power conversion or matching components between the fuel cell stack, battery and/or load. A less costly, less complex and/or more efficient approach is desirable.
In one aspect, a fuel cell system includes: a fuel cell stack, a battery, a series pass element electrically coupled between at least a portion of the fuel cell stack and a portion of the battery, a regulating circuit for regulating current through the series pass element in response to a greater of a battery charging current error, a battery voltage error, and a stack current error, a reactant delivery system for delivering reactant to the fuel cells, the reactant delivery system including at least a first control element adjustable to control a partial pressure in a flow of a reactant to at least some of the fuel cells, and a control circuit coupled to receive signals corresponding to a voltage on an input side and an output side of the series pass element and configured to determine a deviation of a voltage difference across the series pass element from a desired operational condition based on the received signals, the control circuit further coupled to control the at least first control element based on the determined deviation. The fuel cell system may include a battery charging current error integrator having a first input coupled to receive a battery charging current signal and a second input coupled to receive a battery charging current limit signal. The fuel cell system may also include a battery voltage error integrator having a first input coupled to receive a battery voltage signal and a second input coupled to receive a battery voltage limit signal. The fuel cell system may further include a stack current error integrator having a first input coupled to receive a stack current signal and a second input coupled to receive a stack current limit signal. The fuel cell system may additionally include an OR circuit for selecting a greater of the battery charging current error, the battery voltage error and the stack current error.
In another aspect, a fuel cell system includes: a number of fuel cells forming a fuel cell stack, a number of battery cells forming a battery, a series pass element, a blocking diode electrically coupled between the fuel cell stack and the series pass element, a regulating circuit for regulating current through the series pass element in proportion to at least a greater of a difference between a battery charging current and a battery charging current limit, a difference between a battery voltage and a battery voltage limit, and a difference between a stack current and a stack current limit, a reactant delivery system for delivering reactant to the fuel cells, the reactant delivery system including at least a first flow regulator adjustable to control a partial pressure in a flow of a reactant to at least some of the fuel cells, and a control circuit coupled to receive signals corresponding to a voltage difference across the series pass element and to provide a control signal to at least the first control element mathematically related to a voltage difference across the series pass element.
In yet another aspect, a circuit for a fuel cell system includes a series pass element electrically coupleable between at least a portion of the fuel cell stack and a portion of the battery, a regulating circuit for regulating current through the series pass element in response to a greater of a battery charging current error, a battery voltage error and a stack current error, and a control circuit coupled to receive signals corresponding to a voltage on an input side and an output side of the series pass element and configured to determine a deviation of a voltage difference across the series pass element from a desired operational condition based on the received signals and to produce a control signal based on the determined deviation.
In a further aspect, a circuit for a fuel cell system includes a series pass element, a blocking diode electrically coupled in series with the series pass element, a regulating circuit coupled to the series pass element to regulate a current through the series pass element in proportion to at least a greater of a difference between a battery charging current and a battery charging current limit, a difference between a battery voltage and a battery voltage limit, and a difference between a stack current and a stack current limit, and a control circuit coupled to receive signals corresponding to a voltage across the serried pass element and to provide a control signal mathematically related to a voltage difference across the series pass element.
In yet a further aspect, a circuit for a fuel cell system includes a battery charging sensor, a battery charging current error integrator, a battery voltage sensor, a battery voltage error integrator, a stack current sensor, a stack current error integrator, an OR circuit coupled to the output of each of the battery current error integrator, the battery voltage error integrator and the stack current error integrator, a series pass element having a pair of terminals for selectively providing a current path and a control terminal coupled to the OR circuit for regulating current through the current path in proportion to a greater of the battery current error signal, the battery voltage error signal and the stack current error signal, and a control circuit coupled to receive signals corresponding to a voltage on an input side and an output side of the series pass element and configured to determine a deviation of a voltage difference across the series pass element from a desired operational condition based on the received signals and to produce a control signal based on the determined deviation.
In even a further aspect, a method of operating a fuel cell system includes: supplying current at a number of output terminals from at least one of a fuel cell stack and a battery electrically coupled in parallel with the fuel cell stack, in a first stage, regulating a current through a series pass element in proportion to at least a greater of a difference between a battery charging current and a battery charging current limit, a difference between a battery voltage and a battery voltage limit, and a difference between the stack current and the stack current limit, and in a second stage, adjusting a partial pressure of a reactant flow to at least a portion of the fuel cell stack to maintain a series pass element at a desired saturation level.
In even a further aspect, a method of operating a fuel cell system includes: determining a battery charging current error, determining a battery voltage error, determining a stack current error, regulating current through the series pass element in response to a greater of the battery charging current error, the battery voltage error, and the stack current error, determining a voltage difference across the series pass element, determining an amount of deviation of the determined voltage difference from a desired operational condition of the series pass element, and for at least one reactant flow to at least a portion of the fuel cell stack, adjusting a partial pressure of the reactant flow based on the determined amount of deviation. Determining the battery charging current error may include integrating a difference between a battery charging current and a battery charging current limit over time. Determining the battery voltage error may include integrating a difference between a battery voltage and a battery voltage limit over time. Determining the stack current error may include integrating a difference between a stack current and a stack current limit over time. The method may also include selecting the greater of the battery charging current error, the battery voltage error and the stack current error, level shifting the selected one of the errors, and applying the level shifted error to a control terminal of the series pass element. The method may further include determining a temperature proximate the battery and determining the battery voltage limit based at least in part on a determined temperature.
In still a further aspect, a method of operating a fuel cell system includes: determining a difference between a battery charging current and a battery charging current limit, determining a difference between a battery voltage and a battery voltage limit, determining a difference between a stack current and a stack current limit, regulating a current through a series pass element in proportion to at least a greater of the difference between the battery charging current and the battery charging current limit, the difference between the battery voltage and the battery voltage limit, and the difference between the stack current and the stack current limit, determining a voltage difference across the series pass element, determining an amount of deviation of the determined voltage difference from a desired operational condition of the series pass element, and for at least one reactant flow to at least a portion of the fuel cell stack, adjusting a partial pressure of the reactant flow based on the determined amount of deviation.
In yet still a further aspect, a combined fuel cell system includes two or more individual fuel cell systems electrically coupled in series and/or parallel combinations to produce a desired current at a desired voltage.